stabilize core parts of MaybeUninit and deprecate mem::uninitialized in the future

Also expand the documentation a bit

Author: RalfJung

src/liballoc/lib.rs

@@ -110,7 +110,7 @@
 #![feature(rustc_const_unstable)]
 #![feature(const_vec_new)]
 #![feature(slice_partition_dedup)]
-#![feature(maybe_uninit, maybe_uninit_slice, maybe_uninit_array)]
+#![feature(maybe_uninit_extra, maybe_uninit_slice, maybe_uninit_array)]
 #![feature(alloc_layout_extra)]
 #![feature(try_trait)]
 #![feature(iter_nth_back)]

src/libcore/lib.rs

@@ -125,7 +125,7 @@
 #![feature(structural_match)]
 #![feature(abi_unadjusted)]
 #![feature(adx_target_feature)]
-#![feature(maybe_uninit, maybe_uninit_slice, maybe_uninit_array)]
+#![feature(maybe_uninit_slice, maybe_uninit_array)]
 #![feature(external_doc)]
 
 #[prelude_import]